There are approaches used for passive hybrid assembly of semiconductor optical components with planar waveguides, such as that used by Blauvelt in U.S. patent application US2004/0052467 that uses evanescent (vertical in this case) coupling to an underlying passive waveguide wafer. This approach is limited to vertical evanescent coupling of components and precludes the straightforward integration of non-waveguiding components such as free space optical isolators and thin film filters. In addition, the alignment technique means that the cladding layer of the waveguide wafer has to be both very thin to enable the evanescent coupling to take place and substantially flat.
An alternative is that disclosed by Maxwell in U.S. Pat. No. 6,778,718 relating to the hybrid integration of active semiconductor components, whereby the top of a planar surface of a waveguide cladding provides the vertical alignment reference frame and mechanical end stops are used to provide lateral positioning. Light is end-fire coupled into the waveguide device through a hole manufactured into the waveguide layer. This approach again talks of a planar surface for the alignment.
In reality, however, the top surface of a cladding over waveguides may not always be planar and in some situations the cladding surface of the waveguide device may have undulations or bumps above the regions which define the waveguide core.
Fasham in GB-A-2,379,995 used an active alignment approach for hybrid integration and there is a view expressed in the documents by Fasham and Blauvelt which maintains that a passive approach for vertical positioning of additional devices placed on a non-planar surface is not possible without additional processing steps to planarise the surface. These undulations would have to be removed through a process such as chemical-mechanical polishing (CMP) to provide a planar surface for alignment, or some level of active alignment would have to be undertaken to compensate for the non-planar surface. In addition, using a planar surface as described by Maxwell and Blauvelt, increases the susceptibility of the alignment process to contamination from dust or dirt on the cladding surface which can compromise the alignment accuracy. In both cases mentioned, the vertical reference frame for alignment is the planar surface of the cladding of the waveguide device, and in both cases, there are limitations associated with the requirements for a planar surface.
The present invention, at least in preferred embodiments, seeks to enable passive alignment techniques to be adopted without the need to planarise the surface of the waveguide.